Coupling device between left and right wheels of vehicle

ABSTRACT

This is a connecting device (7) to be interposed between left and right wheels (5L, 5R) of a vehicle. It enables to perform a starting assistance control, a cornering control to improve the cornering performance by positively generating a difference rotation between left and right wheels, and a differential limiting control for restricting the difference rotation between the left and right wheels. There are provided: a differential gear (8) having a first rotational element (8a), and second and third rotational elements (8b, 8c) so arranged that, when one of them rotates in normal direction relative to the first rotational element (8a), the other thereof rotates in the reverse direction; and a driving source (9) connected to the first rotational element (8a). One of the second and third rotational elements, e.g., the third rotational element (8c) is coupled to one (5R) of the rear wheels via the first power transmission system (10), and the second rotational element (8b) is coupled to the other (5L) of the rear wheels via second and third power transmission systems (11, 12) which are selectively established via a switching means (13). The gear ratios of both the first and second power transmission systems (10, 11) are identically set, and the gear ratio of the third power transmission system (12) is set so as to be opposite in direction to, but be equal in absolute value to, the above-mentioned gear ratio.

TECHNICAL FIELD

The present invention relates to a coupling device to be providedbetween left and right wheels of a vehicle, in particular between theleft and right idler wheels that are not driven by an engine.

BACKGROUND ART

Conventionally, as this kind of coupling device, there is known, inPublished Unexamined Patent Application No. 131855/1993, a couplingdevice in which: between the left and right wheels there are provided afirst coupling passage which transmits the rotation of one of the leftand right wheels to the other thereof with increasing speed, and asecond coupling passage which transmits the rotation of one of thewheels to the other thereof with decreasing speed; in both the couplingpassages a clutch is interposed respectively; and at the time ofcornering when one of the wheels becomes an inner wheel, the clutch forthe first coupling passage is engaged, and at the time of cornering whenone of the wheels becomes an outer wheel, the clutch for the secondcoupling passage is engaged so that the outer wheel is positivelyspeeded up as compared with the inner wheel. In this manner, bygenerating a braking force on the inner wheel side and a driving forceon the outer wheel side, the cornering performance is sought to beimproved.

By the way, at the time of starting on a slippery road, such as on asnowy road, or the like, it is being considered to perform a startingassistance by driving the idler wheels by a driving source, such as anelectric motor, or the like. It is desired to obtain a startingassistance function in addition to the above-described cornering controlfunction in that the cornering performance is improved by generating adifference rotation between the left and right wheels.

The present invention has an object of providing a compact andinexpensive coupling device which meets the above-described desire andin which a difference in rotation between the right and left wheels canbe generated by utilizing the driving source for the startingassistance.

SUMMARY OF INVENTION

In order to solve the above-described problem, according to a firstfeature of the present invention, a coupling device to be providedbetween left and right wheels of a vehicle is arranged to comprise: adifferential gear having a first rotational element, and second andthird rotational elements one of which rotates in normal direction whilethe other thereof rotates in reverse direction, respectively, relativeto the first rotational element; a driving source which is coupled tothe first rotational element of the differential gear; a first powertransmission system which couples the second rotational element to oneof the left and right wheels; and a pair of second and third powertransmission systems which are selectively established via switchingmeans and which couple the third rotational element to the other of theleft and right wheels; wherein gear ratios of the first powertransmission system and the second power transmission system are bothset to an identical first gear ratio; and wherein a gear ratio of thethird power transmission system is set to a second gear ratio which isdifferent from the first gear ratio.

At the time of starting of the vehicle, the driving source is rotated innormal direction or in reverse direction in a state in which the secondpower transmission system is established. According to this arrangement,the driving force is equally transmitted from the differential gear tothe left and right wheels via the first and second power transmissionsystems of the mutually identical gear ratio. The left and right wheelsare both rotated either in normal direction or in reverse direction,whereby the starting assistance for forward running or reverse runningis performed.

After starting, the third power transmission system is established andalso the driving source is stopped. In this case, if the second gearratio that is the gear ratio of the third power transmission system isset so as to be opposite in direction to, but be equal in absolute valueto, the gear ratio of the first power transmission system, the firstrotational element of the differential gear will cease to rotate as longas the left and right wheels are rotating at the same speed. Therefore,when the rotation of the first rotational element is restricted bybraking the driving source, the differential limiting between the leftand right wheels is performed, whereby the straight-running stability isimproved.

Further, when the diving source is rotated in normal direction in astate in which the third power transmission system is established, oneof the left and right wheels is speeded up relative to the other thereofand, when the driving source is rotated in the reverse direction, saidthe other of the left and right wheels is speeded up relative to saidone of the left and right wheels. Therefore, if the driving source isrotated in normal direction or reverse direction such that the wheelwhich becomes an outer wheel at the time of cornering is speeded up, adriving force is added to the outer wheel and a braking force is addedto the inner wheel, whereby a yawing moment in the cornering directionis generated, resulting in an improvement in the cornering performance.

In the above-described arrangement, the number of the differential gearis selected to be one and the second and third power transmissionsystems are selectively established. However, a similar function canalso be obtained by using two differential gears. Therefore, accordingto a second feature of the present invention, a coupling device to beprovided between left and right wheels of a vehicle, is arranged tocomprise: a pair of first and second differential gears each having afirst rotational element, and second and third rotational elements, oneof which rotates in normal direction while the other thereof rotates inreverse direction, respectively, relative to the first rotationalelement; a driving source which is selectively coupled via switchingmeans to the first rotational element of the first differential gear andto the first rotational element of the second differential gear; whereinthe second rotational element and the third rotational element of thefirst differential gear are coupled at a first gear ratio to oneanother, respectively, of the left and right wheels; wherein the secondrotational element of the second differential gear is coupled to one ofthe left and right wheels at a second gear ratio; and wherein the thirdrotational element of the second differential gear is coupled to saidthe other of the left and right wheels at a third gear ratio which isdifferent from the second gear ratio.

In this arrangement, when the driving source is coupled to the firstrotational element of the first differential gear, the same state isattained as at the time when the second power transmission system isestablished in the above-described first feature. If the driving sourceis rotated in normal or reverse direction in this state at the time ofstarting, the driving force is equally transmitted to the left and rightwheels, whereby the starting assistance for forward running or reverserunning is performed.

Further, when the driving source is coupled to the first rotationalelement of the second differential gear, the same state is attained asat the time when the third power transmission system is established inthe above-described first feature. Then, if the third gear ratio is setso as to be opposite in direction to, but equal in absolute value to,the second gear ratio, the first rotational element of the seconddifferential gear will not rotate as long as the left and right wheelsare rotating at the same speed. Therefore, if the rotation of the firstrotational element of the second differential gear is restricted bybraking the driving source, the differential limiting between the leftand right wheels is performed, whereby the straight-running stability isimproved. Further, at the time of cornering, the outer wheel is speededup by rotating the driving source in normal or reverse direction toimprove the cornering performance.

As described above, according to the present invention, in either of thefirst feature and the second feature, there can be obtained a compactand inexpensive coupling device which can perform the startingassistance control, the cornering control, and further the differentiallimiting control by an organic combination of the differential gear andthe driving source. In particular, since an arrangement is made suchthat the difference in rotation between the left and right wheels istaken out by the differential gear, the rotation speed required for thecornering control and the differential limiting control becomes low.Therefore, even if the output torque of the driving source is small, thetorque required for the cornering control and the differential limitingcontrol can be obtained by performing a sufficient speed reduction.Consequently, the driving source can be made smaller, and it becomescompact.

By the way, in a state in which the third power transmission system isestablished in the first feature, and in a state in which the drivingsource is coupled to the first rotational element of the seconddifferential gear in the second feature, if there occurs a differencerotation between the left and right wheels, the first rotational elementof the differential gear (the second differential gear in the secondfeature) rotates at a speed corresponding to the difference rotation,whereby the driving source is driven in an inverse manner. It followsthat, if an excessive difference rotation occurs between the left andright wheels due, for example, to locking of one wheel at the time ofbraking, or the like, the driving source will rotate excessively tothereby impair the durability of the driving source.

In this case, if there is provided clutch means which releases thecoupling between the left and right wheels or the coupling between thedifferential gear (both the first and second differential gears in thesecond feature) and the driving source when the difference rotationbetween the left and right wheels has exceeded a predetermined value, anexcessive rotation of the driving source due to the excessive differencerotation can be prevented. This is advantageous in improving thedurability of the driving source.

By the way, if the switching means in the first feature is arranged tobe switchable to a neutral state in which neither of the second andthird power transmission systems is established, and if the switchingmeans in the second feature is arranged to be switchable to a neutralstate in which the driving source is coupled to neither of the firstrotational element of one of the differential gears and the firstrotational element of the other of the differential gears such that theswitching means is switched to the neutral state when the differencerotation between the left and right wheels has exceeded a predeterminedvalue, the excessive rotation of the driving source can be preventedeven if the clutch means is not separately provided.

By the way, in the above-describe first and second features, the drivingsource is selected to be one in number. According to a third feature ofthe present invention, a coupling device to be provided between left andright wheels of a vehicle is arranged to comprise: a pair ofdifferential gears each having first, second, and third rotationalelements; and a pair of driving sources each being respectively coupledto the first rotational element of each of the differential gears;wherein the second rotational element of each of the differential gearsis coupled to each of the left and right wheels; wherein the thirdrotational elements of both the differential gears are coupled to eachother; and wherein brake means which is capable of restricting therotation of the third rotational element is provided.

At the time of starting the vehicle, the brake means is operated andalso both the driving sources are rotated both in the normal directionor in the reverse direction. According to this arrangement, the thirdrotational elements of both the differential gears function as reactionforce receivers. The driving forces of both the driving sources aretransmitted to the second rotational elements via the first rotationalelements of both the differential gears, whereby the left and rightwheels are rotated both in the normal direction or in the reversedirection, and the starting assistance in the forward running or reverserunning is performed.

After starting, the brake means is released and also both the drivingsources are stopped. In this case, if both the driving sources are keptbraked, the first rotational elements of both the differential gearsoperate as reaction force receivers, and a state is attained in whichthe left and right wheels are directly coupled to each other via thesecond rotational element and the third rotational element of both thedifferential gears. The difference in rotation between the left andright wheels is thereby restricted and the straight running stability isimproved.

Further, at the time of cornering, the driving source to be connected tothe differential gear on the side of the wheel that becomes the outsidewheel is rotated in the normal direction and the driving source to beconnected to the differential gear on the side of the wheel that becomesthe inside is rotated in the reverse direction. According to thisarrangement, the outside wheel is speeded up and also the inside wheelis retarded, whereby the cornering performance is improved.

As described above, also in the third feature, by an organic combinationof the differential gear and the driving source, there can be obtained acompact and inexpensive coupling device in which the starting assistancecontrol and the cornering control and further the differential limitingcontrol can be performed. Particularly, since an arrangement is madesuch that the difference in rotation between the left and right wheelsis taken out by the differential gears, the rotation speed required forthe cornering control and the differential limiting control becomessmall. Therefore, even if the output torque of the driving source issmall, the torque required for the cornering control and thedifferential limiting control can be obtained by performing a sufficientspeed reduction. Consequently, the driving source can be made small, andit becomes compact.

By the way, in order not to generate a rotational speed differencebetween the front and rear wheels, it is desirable to make uniform theaverage speed of the inner and outer wheels by making equal thespeeding-up amount of the outer wheel and the retardation amount of theinner wheel. In this case, if the dimensions of each of the rotationalelements of both the differential gears are respectively made equal toeach other, the speeding-up amount of the outer wheel and theretardation amount of the inner wheel can be made equal to each othersimply by rotating one of the driving sources and the other of thedriving sources in the normal direction and the reverse direction at anequal speed. There is therefore an advantage in that the control of thedriving sources becomes easy.

Further, it is also considered: to constitute each of the differentialgears by a bevel gear type of differential gear in which a pair of sidegears made up of bevel gears and a pinion to be engaged with both theside gears are rotatably supported on a differential case; and to usethe differential case, one of the side gears, and the other of the sidegears, respectively, as the above-described first, second, and thirdrotational elements. However, in this arrangement, if the differentialcase which serves as the first rotational element is rotated by thedriving source in a state in which the rotation of said the other of theside gears which serves as the third rotational element is restricted bythe brake means, said one of the side gears which serves as the secondrotational element is rotated by being speeded up, whereby the drivingtorque to be transmitted to the wheels at the time of starting islowered. Even in case the differential case is arranged to be the thirdrotational element and said the other of the side gears is arranged tobe the first rotational element, said one of the side gears which servesas the second rotational element will be rotated at the same rotationalspeed as the first rotational element. Therefore, in order to obtain thedriving torque required for the starting assistance, either a drivingsource having a larger output torque must be used as the driving source,or a reduction gear having a larger reduction gear ratio must beseparately added.

On the contrary, if each of the differential gears is constituted by aplanetary gear type of transmission comprising a sun gear, a ring gear,and a carrier for supporting a planetary gear to be engaged with boththe sun gear and the ring gear, and if the sun gear is used as the firstrotational element, and the carrier is used as the second rotationalelement, and the ring gear is used as the third rotational element, whenthe sun gear which serves as the first rotational element is driven bythe driving source in a state in which the rotation of the ring gearwhich serves as the third rotational element is restricted by the brakemeans, the carrier which serves as the second rotational element will berotated at a lower speed in a predetermined reduction gear ratio.Therefore, as the driving source a smaller one having a smaller outputtorque can be used. In addition, even in case a reduction gear is added,a reduction gear having a smaller reduction gear ratio will suffice,which is advantageous.

By the way, when an excessive difference in rotation occurs between theleft and right wheels due, for example, to the locking of one wheel atthe time of braking, or the like, the driving source will be driven inan inverse manner at a speed corresponding to this difference inrotation, whereby the durability of the driving source is impaired. Inthis case, if there is provided clutch means which releases the couplingbetween the left and right wheels or the coupling between at least oneof both the differential gears and the corresponding one of the drivingsources when a difference in rotation between the left and right wheelshas exceeded a predetermined value, the driving in inverse an manner ofthe driving source due to the difference in rotation is prevented. Thisis advantageous in improving the durability of the driving source.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing an example of using the device of thepresent invention.

FIG. 2 is a skeleton diagram showing a first embodiment of the device ofthe present invention.

FIG. 3 is a flow chart showing a control program of the firstembodiment.

FIG. 4 is a flow chart showing a sub-routine for the starting assistancecontrol.

FIG. 5 is a skeleton diagram showing a second embodiment of the deviceof the present invention.

FIG. 6 is a flow chart showing a control program of the secondembodiment.

FIG. 7 is a skeleton diagram showing a third embodiment of the device ofthe present invention.

FIG. 8 is a skeleton diagram showing a fourth embodiment of the deviceof the present invention.

FIG. 9 is a skeleton diagram showing a fifth embodiment of the device ofthe present invention.

FIG. 10 is a skeleton diagram showing a sixth embodiment of the deviceof the present invention.

FIG. 11 is a skeleton diagram showing a seventh embodiment of the deviceof the present invention.

FIG. 12 is a skeleton diagram showing an eighth embodiment of the deviceof the present invention.

PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a front-wheel-drive vehicle in which the left and rightfront wheels 3L, 3R are driven by an engine 1 via a transmission 2. Eachof the front wheels 3L, 3R is coupled to the transmission 2 via eachdrive shaft 4L, 4R which has respectively constant velocity joints 4a onboth ends.

To the left and right rear wheels 5L, 5R, which are idler wheels, thereare connected coupling shafts 6L, 6R which have respectively constantvelocity joints 6a on both ends, and a coupling device 7 is interposedbetween both the coupling shafts 6L, 6R.

The coupling device 7 is provided, as shown in FIG. 2, with adifferential gear 8 and a driving source 9. The differential gear 8 isconstituted by a bevel gear type of differential gear which is made upby rotatably supporting, on a differential gear case 8a, a pair of leftand right side gears 8b, 8c comprising bevel gears, and a pair ofpinions 8d which are geared with both the side gears 8b, 8c. And a gear9b on an output shaft 9a of the driving source 9 is geared with a ringgear 8e which is fixed to the differential gear case 8a, to therebycouple the driving source 9 to the differential gear case 8a. Also, oneof the left and right side gears, e.g., the side gear 8c on the rightside, is coupled to the constant velocity joint 6a on the coupling shaft6R for the right rear wheel 5R via a first power transmission system 10.Further, the side gear 8b on the left side is coupled to the constantvelocity joint 6a on the coupling shaft 6L for the left rear wheel 5Lvia second and third power transmission systems 11, 12 which areselectively established via a switching means 13.

The first and second power transmission systems 10, 11 are constitutedby gear trains which are made up of driving gears 10a, 11a and drivengears 10b, 11b. The gear ratios of both the power transmission systems10, 11 are set to a mutually identical first gear ratio r1. The thirdpower transmission system 12 is constituted by a gear train which ismade up of a driving gear 12a, an intermediate idle gear 12b, and adriven gear 12c. Its gear ratio is set to a second gear ratio r2 (=-r1)that is opposite in direction to, but is equal in absolute value to, thefirst gear ratio r1.

By the way, the following arrangement may also be employed. Namely, thedifferential gear 8 is constituted by a planetary type of differentialgear comprising a sun gear, a ring gear, and a carrier for supporting apinion which is geared with both the sun gear and the ring gear, and thedriving source 9 is connected to the carrier. And one of the sun gearand the ring gear, e.g., the sun gear is connected to one of the leftand right rear wheels 5L, 5R, e.g., the right rear wheel 5R, via thefirst power transmission system 10, and the ring gear is connected tothe left rear wheel 5L via the second and third power transmissionsystems 11, 12. In this case, when the sun gear is rotated in normal orreverse direction of rotation relative to the carrier, the ring gearrotates in a direction opposite to that of the sun gear at a relativerotational speed of v/rp relative to the carrier, where v is therelative rotational speed of the sun gear relative to the carrier, andrp is the gear ratio between the ring gear and the sun gear (number ofteeth of the ring gear/number of teeth of the sun gear). Therefore, thegear ratio between the driven gear 11b and the driving gear 11a of thesecond power transmission system 11 (number of teeth of the drivengear/number of teeth of the driving gear) is set to r1/rp, and the gearratio between the entire second power transmission system 11 inclusiveof the ring gear and the sun gear is set to r1. Similarly, the gearratio between the driven gear 11c and the driving gear 11a of the thirdpower transmission system 12 is also set to r1/rp.

The driving source 9 is constituted by an electric motor which containstherein a two-stage planetary gear type of reduction gear, and iscontrolled by a controller 14 via a driver circuit 15. Though notillustrated, the driver circuit 15 contains therein a conventionalcircuit for switching the rotation to normal or reverse direction and aregeneration brake circuit. By the way, it is also possible toconstitute the driving source 9 by one which is other than the electricmotor, e.g., a hydraulic motor.

The switching means 13 is constituted by a dog clutch which is providedon a gear shaft of the side gear 8b in a position between the drivinggears 11a, 12a of both the second and third power transmission systems11, 12 and which has an axially movable dog member 13a. It is thus soarranged that the driving gears 11a, 12a of both the power transmissionsystems 11, 12 can be selectively coupled to the side gear 8b by movingthe dog member 13a by means of a solenoid 13b to be controlled by thecontroller 14. By the way, it may also be so arranged that the drivinggears 11a, 12a of both the second and third power transmission systems11, 12 are directly coupled to the side gear 8b to thereby selectivelycouple the driven gears 11b, 12c of both the power transmission systems11, 12 to the constant velocity joint 6a via the switching means 13.Further, it may also be so arranged that separate clutches made up ofdog clutches, electromagnetic clutches, or the like, are interposed inthe second power transmission system 11 and the third power transmissionsystem 12 to thereby selectively establish one of the correspondingpower transmission systems by engaging one of both the clutches.However, in order to simplify the switching means 13, the illustratedexample, which uses one common dog clutch, is appropriate.

To the controller 14 there are inputted signals of wheel speed sensors16L, 16R which detect the speeds of the left and right front wheels 3L,3R, wheel speed sensors 17L, 17R which detect the speeds of the left andright rear wheels 5L, 5R, a brake lamp switch 18, a side brake switch19, a back light switch 20, and a steering angle sensor 21.

The controller 14 receives a signal from a main controller (notillustrated) which controls the engine 1 and the transmission 2, andmakes a determination as to whether or not the vehicle is in a startingoperation or not (S1), performs a starting assistance control (S2) atthe time of starting and, after starting, performs a cornering controland a differential limiting control (S3) as shown in FIG. 3.

Details of the starting assistance control are as shown in FIG. 4.First, a determination is made as to the presence or absence ofabnormality in the system (Q1). If there is no abnormality, acomputation is made of the difference ΔV between the front wheel speedVf and the rear wheel speed Vr (Q2), and a determination is made as towhether ΔV exceeds a set value ΔVS or not (Q3). It is when the frontwheels as the driving wheels are slipping that the condition of ΔV>ΔVSis satisfied. In this case, the starting assistance becomes necessaryuntil the rear wheel speed Vr, i.e., the vehicle speed exceeds a setvalue VrS (e.g., 15 km/h). On the other hand, when the brake is beingapplied, it is useless to perform the starting assistance. Therefore,when ΔV>ΔVS, a determination is then made as to whether the threeconditions in that the rear wheel speed Vr is below VrS, that the brakelamp is OFF, and that the side brake switch is OFF are satisfied or not(Q4, Q5, Q6). When the three conditions are satisfied, a determinationis made as to whether the back light is OFF or not (Q7). When the backlight is OFF, namely, at the time of forward running, the second powertransmission system 11 is established by the switching means 13 and alsothe driving source 9 is rotated in the normal direction (Q8). When theback light is ON, namely, at the time of reverse running, the secondpower transmission system 11 is established by the switching means 13and also the driving source 9 is rotated in the reverse direction (Q9).

Here, once the second power transmission system 11 has been established,the left and right side gears 8b, 8c of the differential gear 8 will becoupled to the left and right rear wheels 5L, 5R at the mutuallyidentical first gear ratio r1. When the differential gear case 8a of thedifferential gear 8 is rotated in the normal direction or in the reversedirection by the driving source 9, the driving force is equallytransmitted to the left and right rear wheels 5L, 5R via the left andright side gears 8b, 8c to thereby perform the starting assistance inthe forward or reverse running.

When there is an abnormality in the system or when the startingassistance is not required, namely, when ΔV≦ΔVS, or when Vr≧VrS, or whenthe brake lamp is ON, or when the side brake switch is ON, the thirdpower transmission system 12 is established by the switching means 13and also the driving source 9 is stopped (Q1O).

After having started, while the switching means 13 is maintained in astate of establishing the third power transmission system 12, thedriving source 9 is rotated in the normal or reverse direction at thetime of cornering control. Namely, at the time of cornering to the left,the driving source 9 is rotated in the normal direction, and at the timeof cornering to the right, the driving source 9 is rotated in thereverse direction. Here, in a state in which the third powertransmission system 12 is established, the side gear 8b on the left sidewill be rotated in the direction opposite to that of the side gear 8c onthe right side. When the left and right rear wheels 5L, 5R are rotatingat the same speed at the time of forward running, the side gear 8b onthe left side is rotated in the reverse direction at a speed equal tothe rotational speed in the normal direction of the side gear 8c on theright side. Since each of the pinions 8d rotates on its own axis in apredetermined position, the differential gear case 8a does not rotate.If the differential gear case 8a is rotated in the normal direction inthis state by the rotation in the normal direction of the driving source9, the side gear 8c on the right side comes to be rotated via thepinions 8d in the normal direction at a speed higher than that of theside gear 8b on the left side. Further, if the differential gear case 8ais rotated in the reverse direction by the rotation in the reversedirection of the driving source 9, the side gear 8b on the left sidecomes to be rotated in the reverse direction via the pinions 8d at aspeed higher than that of the side gear 8c on the right side. In thisway, when the driving source 9 is rotated in the normal direction at thetime of cornering to the left, the right rear wheel 5R is positivelyspeeded up to thereby generate the difference rotation between the leftand right rear wheels 5L, 5R. As a result, the driving force is added tothe right rear wheel 5R which is the outer wheel and the braking forceis added to the left rear wheel 5L which is the inner wheel. A yawingmoment in the direction of cornering to the left is thus generated tothereby improve the left-cornering performance. If the driving source 9is rotated in the reverse direction at the time of cornering to theright, the left rear wheel 5L which is the outer wheel is positivelyspeeded up, resulting in an improvement in the right-corneringperformance.

By the way, in performing the cornering control, a target corneringradius of the vehicle is computed from a vehicle speed and a steeringangle, and the driving source 9 is controlled such that the value of thedifference rotation between the left and right rear wheels 5L, 5R fallswithin this target cornering radius.

The differential limiting control is performed at the time of straightrunning or of cornering at a high speed by braking the driving sourcethrough the closure of the regenerating brake circuit. According to thisoperation, the rotation of the differential gear case 8a is restrictedand therefore the left and right side gears 8b, 8c come to be rotated atthe same speed in the directions opposite to each other via the pinions8d. Thus, the difference rotation between the left and right rear wheels5L, 5R is limited by the braking force of the driving source 9. As aresult, when a yawing moment is applied to the vehicle by disturbancesor the like, a moment to resist it is generated and the stability instraight running and the stability in cornering at a high speed areimproved.

By the way, when a difference rotation occurs between the left and rightrear wheels 5L, 5R, the differential gear case 8a is rotated at a speedcorresponding to the difference rotation to thereby drive, in inversemanner, the driving source 9. Therefore, if an excessive differencerotation is generated between the left and right rear wheels 5L, 5R dueto locking of one of the wheels at the time of braking, or the like, thedriving source 9 will be excessively rotated, resulting in a bad effecton durability.

As a solution, in an example shown in FIG. 5, a clutch means 22 isinterposed between the left and right rear wheels 5L, 5R, e.g., betweenthe differential gear 8 and the first power transmission system 10. Whenthe difference rotation between the left and right rear wheels 5L, 5Rhas exceeded a predetermined value, the clutch means 22 is disengaged tothereby release the coupling between the differential gear 8 and thefirst power transmission system 10. According to this arrangement, thedriving, in inverse manner, of the driving source 9 due to thedifference rotation between the left and right rear wheels 5L, 5R willno longer occur. The excessive rotation of the driving source 9 due tothe excessive difference rotation is thereby prevented, and thedurability of the driving source 9 is improved.

The clutch means 22 is constituted by a dog clutch comprising astationary dog member 22a which is coupled to the side gear 8c of thedifferential gear 8, and a movable dog member 22b which is provided onthe drive gear 10a of the first power transmission system 10 in a mannernot relatively rotatable but is movable axially back and forth. Theclutch means 22 is switched on and off by engaging the movable dogmember 22b with, or disengaging it from, the stationary dog member 22aby a solenoid 22c which is controlled by the controller 14.

As shown in FIG. 6, the controller 14 performs the starting assistancecontrol, the cornering control, and the differential limiting control ina similar manner as described above (S1, S2, S3). Also, it makes adetermination as to whether the difference rotation between the left andright rear wheels 5L, 5R has exceeded a predetermined value or not (S4).When the difference rotation is below the predetermined value, theclutch means 22 is switched on (S5). When the difference rotation isabove the predetermined value, the clutch means 22 is switched off (S6).

By the way, as the clutch means 22 an electromagnetic clutch or ahydraulic clutch may also be used. Further, the clutch means 22 may alsobe provided, as shown in FIG. 7, between the differential gear 8 and thedriving source 9, e.g., on the output shaft 9a of the driving source 9.In this case, the clutch means 22 may be arranged to be switched on andoff by the controller 14 in the same manner as in the above-describedembodiment. However, during normal running, the driving source 9 is notdriven in inverse manner unless a difference rotation is generatedbetween the left and right rear wheels 5L, 5R. Therefore, an arrangementmay also be made such that the clutch means 22 is constituted by acentrifugal clutch which is normally switched on, and that the clutchmeans 22 is switched off when the driving source 9 is driven in inversemanner at above a predetermined speed due to an excessive differencerotation between the left and right rear wheels 5L, 5R.

Further, the excessive rotation of the driving source 9 can also beprevented if the switching means 13 is arranged to be switchable to aneutral state in which neither of the second and third powertransmission systems 11, 12 is established such that the switching means13 is switched to the neutral state when the difference rotation betweenthe left and right rear wheels 5L, 5R has exceeded a predeterminedvalue. According to this arrangement, the clutch means 22 becomesneedless and the construction can be simplified.

FIG. 8 shows still another embodiment. In this embodiment, there areprovided a pair of first and second differential gears 81, 82. The leftand right side gears 8b, 8c of the first differential gear 8₁ arerespectively coupled to the left and right rear wheels 5L, 5R at amutually identical first gear ratio. And also one of the side gears ofthe second differential gear 8₂, e.g., the side gear 8c on the rightside, is coupled to the right rear wheel 5R at a second gear ratio, andthe side gear 8b on the left side of the second differential gear 8₂ iscoupled to the left rear wheel 5L at a third gear ratio that is oppositein direction to, but is equal in absolute value to, the second gearratio.

By the way, in the illustrated embodiment, each of the left and rightside gears 8b, 8c of the first differential gear 8₁ is directly coupledto the constant velocity joints on each of the coupling shafts 6L, 6Rfor each of the rear wheels 5L, 5R, and the above-described first gearratio becomes "1." Further, the side gear 8c on the right side of thesecond differential gear 8₂ is coupled to the constant velocity joints6a on the coupling shaft 6R for the right rear wheel 5R via a gear train23 which is made up of a driving gear 23a and a driven gear 23b. Theabove-described second gear ratio is the same as the gear ratio betweenthe driving gear 23a and the driven gear 23b. The side gear 8b on theleft side of the second differential gear 8₂ is coupled to the constantvelocity joint 6a on the coupling shaft 6L for the left rear wheel 5Lvia a gear train which is made up of a driving gear 24a, an intermediateidle gear 24b, and a driven gear 24c. The gear ratio between the drivinggear 24a and the driven gear 24c is arranged to be equal to the gearratio between the driving gear 23a and the driven gear 23b.

On the output side of the driving source 9, there is provided aswitching means 25. This switching means 25 is constituted by that dogclutch provided on the output shaft 9a of the driving source 9 whichcomprises: a dog gear 25a which is engaged with a ring gear 8e fixed tothe differential gear case 8a of the first differential gear 8₁ ; a doggear 25b which is engaged with a ring gear 8e fixed to the differentialgear case 8a of the second differential gear 8₂ ; and an axially movabledog member 25c which is selectively engaged with both the dog gears 25a,25b. By moving the dog member 25c by a solenoid 25d to be controlled bythe controller 14, the driving source 9 is arranged to be selectivelycoupled to the differential gear case 8a of each of the first and seconddifferential gears 8₁, 8₂.

Then, by controlling the solenoid 25d with the controller 14, thedriving source 9 is coupled, in performing the starting assistancecontrol, to the differential gear case 8a of the first differential gear8₁. After starting, the driving source 9 is coupled to the differentialgear case 8a of the second differential gear 8₂. When the driving source9 is coupled to the differential gear case 8a of the first differentialgear 8₁, there will be attained the same state as at the time ofestablishing the second power transmission system 11 in theabove-described embodiment. Therefore, by rotating the driving source 9in the normal direction or in the reverse direction, the driving poweris equally transmitted to the left and right rear wheels 5L, 5R via thefirst differential gear 8₁, whereby the starting assistance at theforward running or the reverse running is performed.

When the driving source 9 is coupled to the differential gear case 8a ofthe second differential gear 8₂, there will be attained the same stateas at the time of establishing the third power transmission system 12 inthe above-described embodiment. Therefore, by rotating the drivingsource 9 to the normal direction at the time of cornering to the left,and by rotating the driving source 9 in the reverse direction at thetime of cornering to the right, the rear wheel that becomes the outerwheel can be speeded up to thereby improve the cornering performance.Further, by braking the driving source 9 at the time of running straightor at the time of cornering at a high speed, the difference rotationbetween the left and right rear wheels 5L, 5R is limited, whereby thestability in running straight and the stability in cornering at a highspeed can be improved.

By the way, as shown in FIG. 9, if a clutch means 22 similar to theabove-described one is interposed between the left and right rear wheels5L, 5R so that the clutch means 22 is switched off when the differencerotation between the left and right rear wheels 5L, 5R has exceeded apredetermined value, an excessive rotation of the driving source 9 dueto an excessive difference rotation can be prevented. Further, theexcessive rotation of the driving source 9 due to the excessivedifference rotation can also be prevented if there is provided a clutchmeans, such as a centrifugal clutch or the like, on the output shaft 9aof the driving source 9 so that the clutch means is switched off whenthe difference rotation has exceeded a predetermined value, therebyreleasing the coupling between both the differential gears 8₁, 8₂ andthe driving source 9. Further, if the switching means 25 is arranged tobe switchable to a neutral state in which the driving source 9 iscoupled to neither the differential gear case 8a of the firstdifferential gear 8₁ nor the differential gear case 8a of the seconddifferential gear 8₂, the excessive rotation of the driving source 9 dueto the excessive difference rotation can be prevented by switching theswitching means 25 to the neutral state when the difference rotation hasexceeded a predetermined value, even if the clutch means 22 is notprovided.

FIG. 10 shows still another embodiment. In this embodiment, the couplingdevice 7 is provided with a pair of left and right differential gears8L, 8R, and a pair of driving sources 9L, 9R. Each of the differentialgears 8L, 8R is constituted by a planetary gear type of differentialgear which comprises a sun gear 80, a ring gear 81, and a carrier 83 forsupporting a planetary gear 82 which is engaged with both the gears 80,81. And each of the driving sources 9L, 9R is coupled to the sun gear 80of each of the differential gears 8L, 8R in a speed-reduction manner viaa reduction gear train 26L, 26R. Also the carrier 83 of each of thedifferential gears 8L, 8R is coupled to the constant velocity joint 6aon each of the coupling shafts 6L, 6R. Further, the ring gears 81 ofboth the differential gears 8L, 8R are coupled together via anintermediate shaft 27. By the way, the dimensions of the sun gears 80,of the ring gears 81, of the planetary gears 82, and of the carriers 83of both the differential gears 8L, 8R are respectively made equal toeach other.

Each of the driving sources 9L, 9R is constituted by a small electricmotor and contains therein a planetary type of reduction gear. It iscontrolled by a controller 14 via respective driver circuits 15L, 15R.By the way, though not illustrated, each of the driver circuits 15L, 15Rcontains therein a conventional switching circuit for switching therotational direction between normal direction and reverse direction, anda regeneration brake circuit. It is also possible to constitute thedriving sources 9L, 9R by ones other than electric motors, e.g., byhydraulic motors.

The coupling device 7 is further provided with a brake means 28 forrestricting the rotation of the ring gears 81 of both the differentialgears 8L, 8R. The brake means 28 is constituted by a dog clutch whichcomprises a stationary dog member 28a which is fixed to an intermediateshaft 27, and a movable dog member 28b which is prevented from rotatingrelative to the casing of the coupling device 7 and which is movableaxially back and forth. The movable dog member 28b is moved back andforth by a solenoid 28c to be controlled by the controller 14 to therebyengage it with, and disengage it from, the stationary dog member 28a,whereby the restriction and releasing of the ring gears 81 areperformed. It is also possible to constitute the brake means 28 by ahydraulic clutch or an electromagnetic clutch which is disposed on thecasing in a relatively non-rotatable manner.

In performing the starting assistance control, the brake means 28 isswitched on by engaging the movable dog member 28b with the stationarydog member 28a, and also both the driving sources 9L, 9R are rotated inthe normal direction (at the time of forward running) or in the reversedirection (at the time of reverse running).

Here, when the brake means 28 is switched on, the ring gears 81 of boththe differential gears 8L, 8R are restricted, whereby the ring gears 81function as reaction force receiving members. When the sun gears 80 ofboth the driving sources 9L, 9R are rotated in the normal or reversedirection, the carrier 83 is rotated in the normal or reverse directionin a speed-reduced state. A required driving torque is thus transmittedto the left and right rear wheels 5L, 5R to thereby perform the startingassistance.

When the system is in disorder or when the starting assistance is notrequired, the brake means 28 is switched off and also both the drivingsources 9L, 9R are stopped.

After starting, while the brake means 28 is kept in a switched-offstate, both the driving sources 9L, 9R are rotated in the normal orreverse direction in performing the cornering control. In more detail,the cornering control is performed, e.g., at the time of cornering tothe right, by rotating the driving source 9L in the normal direction andby rotating the driving source 9R in the reverse direction. When thedriving source 9L is rotated in the normal direction, the carrier 83 ofthe sun gear 80 of the differential gear 8L is rotated in the normaldirection relative to the ring gear 81 due to the rotation in the normaldirection of the sun gear 80 of the driving source 9L. Further, when thedriving source 9R is rotated in the reverse direction, the carrier 83 ofthe sun gear 80 of the differential gear 8R is rotated in the reversedirection relative to the ring gear 81 due to the rotation in thereverse direction of the sun gear 80 of the driving source 9R. In thiscase, a reaction force in the reverse direction is operated on the ringgear 81 of the differential gear 8L, and a reaction force in the normaldirection is operated on the ring gear 81 of the differential gear 8R.However, since both the ring gears 81, 81 are coupled to each other,both the reaction forces are mutually canceled. Accordingly, on thebasis of the rotational speeds of both the ring gears 81, 81, thecarrier 83 of the differential gear 8L, i.e., the left rear wheel 5L, isspeeded up and the carrier 83 of the differential gear 8R, i.e., theright rear wheel 5R, is retarded. As a result, there occurs a differencein rotation between the left and right rear wheels 5L, 5R. And a drivingforce is added to the left rear wheel 5L which is the outer wheel, and abraking force is added to the right rear wheel 5R which is the innerwheel, whereby a yawing moment in the direction of right cornering isgenerated, resulting in an improvement in the right corneringperformance.

Further, since the dimensions of the corresponding rotational elements,such as of the sun gears 80 and of the ring gears 81 of both thedifferential gears 8L, 8R, are the same, the amount of speeding up ofthe left rear wheel 5L and the amount of retardation of the right rearwheel 5R become equal to each other if the absolute values of rotationalspeeds of both the driving sources 9L, 9R are made equal to each other.The average speeds of the left and right rear wheels 5L, 5R thus becomeequal to the speed of the vehicle, whereby the slip of the wheels due tothe difference in rotation between the front and rear wheels can beprevented. And in order to make equal to each other the absolute valuesof the rotational speeds of both the driving sources 9L, 9R, it isenough to make equal the absolute values of the electric current valuesto be charged to both the driving sources 9L, 9R. The control of thedriving sources 9L, 9R therefore becomes easy.

At the time of cornering to the left, the left rear wheel 5L, which isthe inner wheel, is retarded by rotating the driving source 9L in thereverse direction, and the right rear wheel 5R, which is the outerwheel, is speeded up by rotating the driving source 9R in the normaldirection.

By the way, at the time of cornering control, a target cornering radiusof the vehicle is computed from the vehicle speed and the steeringangle, and then both the driving sources 9L, 9R are controlled such thatthe value of the difference rotation between the left and right rearwheels 5L, 5R becomes a value which fits to this target corneringradius.

The differential limiting control is made, at the time of runningstraight or cornering at a high speed, by braking both the drivingsources 9L, 9R through closure of the regeneration brake circuit.According to these operations, the sun gears 80 of both the differentialgears 8L, 8R function as reaction force receivers. The rotation of theleft rear wheel 5L is thus transmitted to the ring gear 81 of thedifferential gear 8L via its carrier 83, and also the rotation of theright rear wheel 5R is transmitted to the ring gear 81 of thedifferential gear 8R via its carrier 83. And, since both the ring gears81, 81 are coupled to each other, the difference, rotation between theleft and right rear wheels 5L, 5R is restricted by the braking force ofboth the driving sources 9L, 9R. As a result, when the yawing moment isadded to the vehicle due to disturbances, or the like, a moment toresist the yawing moment occurs, whereby the stability of runningstraight and the stability of cornering at a high speed are improved.

By the way, if an excessive difference in rotation occurs between theleft and right rear wheels 5L, 5R due to locking of one wheel at thetime of braking, or the like, both the driving sources 9L, 9R are drivenin inverse manner in the normal direction and in the opposite direction,respectively, at an excessive speed due to a function which is oppositeto that at the time of cornering control. A bad effect will thus begiven to the durability of the driving sources 9L, 9R.

As a solution, in an embodiment shown in FIG. 11, an intermediate shaft27 to couple the ring gears 81 of both the driving sources 8L, 8R isdivided, and a clutch means 22 is provided in the divided portion. Boththe differential gears 8L, 8R are thus coupled via the clutch means 22so that, when the difference in rotation between both the left and rightrear wheels 5L, 5R has exceeded a predetermined value, the clutch means22 is switched off to thereby release the coupling between both thedifferential gears 8L, 8R. According to this arrangement, the driving inan inverse manner of both the driving sources 9L, 9R due to thedifference in rotation between the left and right rear wheels 5L, 5Rwill no longer occur. An excessive rotation of both the driving sources9L, 9R due to the excessive difference in rotation is prevented, and thedurability of both the driving sources 9L, 9R is improved.

In this embodiment, the clutch means 22 is constituted by a dog clutchcomprising a stationary dog member 22a which is fixed to one half of thedivided intermediate shaft 27, and a movable dog member 22b which isprovided on the other half of the intermediate shaft 27 so as to berelatively non-rotatable but be axially movable back and forth. Theclutch means 22 is switched on and off by engaging the movable dogmember 22b with, or disengaging it from, the stationary dog member 22aby means of a solenoid 22c to be controlled by the controller 14.

As the clutch means 22, an electromagnetic clutch or a hydraulic clutchmay also be used. Further, in the embodiment shown in FIG. 11, theclutch means 22 is interposed between both the deferential gears 8L, 8R.However, it may also be interposed between one of the deferential gearsand the corresponding one of the rear wheels, e.g., between thedifferential gear 8L and the constant velocity joint 6a on the couplingshaft 6L for the left rear wheel 5L. What is important is that it may beinterposed in a position in which the coupling between the left andright rear wheels 5L, 5R can be released.

Further, also when the coupling between at least one of the differentialgears and the corresponding one of the driving sources is released,there will neither occur the driving in inverse manner of both thedriving sources 9L, 9R due to the difference rotation between the leftand right rear wheels 5L, 5R. Therefore, as shown in FIG. 12, the clutchmeans 22 may also be interposed between the sun gear 80 of one of thedifferential gears, e.g., the differential gear 8L, and the drivingsource 9L. In this case, the clutch means 22 may be made of a type to becontrolled for switching on and off by the controller 14 like in theabove-described one. However, since the sun gear 80 of each of thedifferential gears 8L, 8R does not rotate, during normal running, unlessa difference rotation occurs between the left and right rear wheels 5L,5R, the clutch means 22 may be constituted by a centrifugal clutch whichis normally kept switched on such that the clutch means 22 is switchedoff when the sun gear 80 rotates at above a predetermined rotationalspeed due to an excessive difference rotation between the left and rightrear wheels 5L, 5R.

An explanation has so far been made about embodiments in which thepresent invention is applied to the connecting device between rearwheels of the front-wheel-drive vehicle. The present invention can,however, also be similarly applied to the coupling device between frontwheels of a rear-wheel-drive vehicle.

We claim:
 1. A coupling device provided between left and right idlerwheels of a vehicle, comprising:a pair of differential gears each havinga first, second, and third rotational elements; and a pair of drivingsources each being coupled to said first rotational element of each ofsaid differential gears; wherein said second rotational element of eachof said differential gears is respectively coupled to each of said leftand right wheels; wherein said third rotational elements of both saiddifferential gears are coupled to each other; and wherein brake meanswhich is capable of restricting the rotation of said third rotationalelement is provided.
 2. A coupling device provided between left andright idler wheels of a vehicle according to claim 1, wherein dimensionsof each of said rotational elements of both said differential gears are,respectively, made equal to each other.
 3. A coupling device providedbetween left and right idler wheels of a vehicle according to either oneof claim 1 or claim 2, wherein each of said differential gears isconstituted by a planetary gear transmission comprising a sun gear, aring gear, and a carrier for supporting a planetary gear to be engagedwith both said sun gear and said ring gear, and wherein said sun gear isused as said first rotational element, and said carrier is used as saidsecond rotational element, and said ring gear is used as said thirdrotational element.
 4. A coupling device provided between left and rightwheels of a vehicle comprising:a pair of differential gears each havingfirst, second, and third rotational elements; and a pair of drivingsources each being coupled to said first rotational element of each ofsaid differential gears; wherein said second rotational element of eachof said differential gears is respectively coupled to each of said leftand right wheels; wherein said third rotational elements of both saiddifferential gears are coupled to each other; wherein brake means whichis capable of restricting the rotation of said third rotational elementis provided; and wherein clutch means is provided which releases thecoupling between the left and right wheels or the coupling between oneof said differential gears and a corresponding one of said drivingsources when a difference in rotation between the left and right wheelshas exceeded a predetermined value.
 5. A coupling device providedbetween left and right wheels of a vehicle comprising:a pair ofdifferential gears each having first, second, and third rotationalelements; and a pair of driving sources each being coupled to said firstrotational element of each of said differential gears; wherein saidsecond rotational element of each of said differential gears isrespectively coupled to each of said left and right wheels; wherein saidthird rotational elements of both said differential gears are coupled toeach other; wherein brake means which is capable of restricting therotation of said third rotational element is provided; wherein each ofsaid differential gears is constituted by a planetary gear type oftransmission comprising a sun gear, a ring gear, and a carrier forsupporting a planetary gear to be engaged with both said sun gear andsaid ring gear, in which said sun gear is used as said first rotationalelement, and said carrier is used as said second rotational element, andring gear is used as said third rotational element; and clutch means isprovided which releases the coupling between the left and right wheelsor the coupling between one of said driving sources corresponding to atleast one of both said differential gears when a rotational differencebetween the left and right wheels has exceeded a predetermined value. 6.A coupling device for installation between left and right wheels of avehicle comprising:a pair of differential gears each having first,second, and third rotational elements; and a pair of driving sourceseach being coupled to said first rotational element of each of saiddifferential gears; wherein said second rotational element of each ofsaid differential gears is respectively coupled to each of said left andright wheels; wherein said third rotational elements of both saiddifferential gears are coupled to each other; wherein brake means, whichis capable of restricting the rotation of said third rotational element,is provided; wherein dimensions of each of said rotational elements ofboth said differential gears are, respectively, made equal to eachother; and clutch means is provided which releases the coupling betweenthe left and right wheels or the coupling between one of saiddifferential gears and a corresponding one of said driving sources whena difference rotation between the left and right wheels has exceeded apredetermined value.
 7. A coupling device for installation between leftand right wheels of a vehicle comprising:a pair of differential gearseach having first, second, and third rotational elements; a pair ofdriving sources each being coupled to said first rotational element ofeach of said differential gears; wherein said second rotational elementof each of said differential gears is respectively coupled to each ofsaid left and right wheels; wherein said third rotational elements ofboth said differential gears are coupled to each other; wherein brakemeans, which is capable of restricting the rotation of said thirdrotational element, is provided; wherein dimensions of each of saidrotational elements of both said differential gears are, respectively,made equal to each other; wherein each of said differential gears isconstituted by a planetary gear type of transmission comprising a sungear, a ring gear, and a carrier for supporting a planetary gear to beengaged with both said sun gear and said ring gear; wherein said sunggear is used as said first rotational element, and said carrier is usedas said second rotational element, and said ring gear is used as saidthird rotational elements; and clutch means is provided which releasesthe coupling between the left and right wheels or the coupling betweenone of said driving sources corresponding to at least one of both saiddifferential gears when a rotational difference between the left andright wheels has exceeded a predetermined value.